Synergistic interplay between photoisomerization and photoluminescence in a light-driven rotary molecular motor

Photoactuators and photoluminescent dyes utilize light to perform mechanical motion and undergo spontaneous radiation emission, respectively. Combining these two functionalities in a single molecule would benefit the construction of advanced molecular machines. Due to the possible detrimental interaction between the two light-dependent functional parts, the design of hybrid systems featuring both functions in parallel remains highly challenging. Here, we develop a light-driven rotary molecular motor with an efficient photoluminescent dye chemically attached to the motor, not compromising its motor function. This molecular system shows efficient rotary motion and bright photoluminescence, and these functions can be addressed by a proper choice of excitation wavelengths and solvents. The moderate interaction between the two parts generates synergistic effects, which are beneficial for lower-energy excitation and chirality transfer from the motor to the photoluminescent dye. Our results provide prospects towards photoactive multifunctional systems capable of carrying out molecular rotary motion and tracking its location in a complex environment.


Materials and characterization methods
All chemicals were purchased from Tokyo Chemical Industry Co. Ltd., Sigma-Aldrich Co.
LLC, Thermo Fisher Scientific Inc., Fluorochem Ltd., unless otherwise stated, and they were used without further purification. Motor, BODIPY, M1 were prepared according to literature procedures 1 3 . 1 H and 13 C nuclear magnetic resonance (NMR) data were collected in CDCl3 and recorded on a Varian Mercury-Plus 400 or a Bruker Avance 600 NMR spectrometer at 298 K unless otherwise indicated. Photostationary state (PSS) studies were performed on a Varian Unity Plus 500 NMR spectrometer. Chemical shifts are given in parts per million (ppm) relative to the residual solvent signal. Multiplets in 1 H NMR spectra are designated as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad). High resolution mass spectrometry was performed on an LTQ Orbitrap XL spectrometer. Steady-state UV/vis absorption spectra were recorded on an Agilent 8453 UV-vis Diode Array System, equipped with a Quantum Northwest Peltier controller, in 10 mm quartz cuvettes. Irradiation experiments were performed using fiber-coupled light-emitting diodes (LEDs) obtained from Thorlabs Inc.
Fluorescence spectra were collected with a JASCO FP-6200 spectrometer. Separation of the enantiomers was performed with Thar SFC system with PDA detector using Chiralpak IA (5 m, 4.6x250mm, 10% methanol in supercritical CO2, 4 ml/min). Circular dichroism (CD) spectra were recorded on a JASCO 810 spectropolarimeter.

Difference absorption spectroscopy
Difference absorption spectroscopy measurements were performed using an UV/Vis/NIR spectrometer (Lambda 900) and two different types of the light source for irradiation in the green or red wavelength region. The green irradiation at 505 nm was provided by a fiber coupled LED (M505F3, Thorlabs) with the maximum power of ~26 mW. The red irradiation at 635 nm was implemented using a 635-nm diode laser (Lasermate Group, Inc.) with a maximum power of 550 mW. The output light source irradiated a 1-cm quartz cuvette, containing a 2-mL solution of the studied compounds dissolved in toluene. For experiments of BODIPY/Motor mixed with PtTPBP or PdPc(OBu)8, all samples were degassed in an argon environment for at least 10 min before the measurements. The irradiation time was fixed for 1 min or 5 min, which depends on specific measurements, and the absorption of the samples was recorded before and 10 secs right after irradiation (from 10 secs up to ~30 min, for time-evolution experiments); all in the dark. Finally, difference absorption (ΔOD) spectra were obtained by subtracting the absorption spectrum before irradiation from the ones after irradiation. As the ΔOD spectra are extremely sensitive to temperature, the room temperature was maintained at ~23 ± 1 ℃ for all measurements.

Time-resolved PL spectroscopy
Time-resolved PL spectroscopy measurements were carried out using a Hamamatsu C5680 streak camera equipped with a Ti:sapphire laser (Mira 900, Coherent) with a central wavelength at 800 nm and repetition rate of 76 MHz. The excitation wavelength at 400 nm was obtained using a second harmonic generator with the input of the Ti:sapphire laser. In contrast, the excitation wavelength at 510 nm was obtained by using a SCG-800 Photonic Crystal Fiber (Newport Corp.) to generate a white light continuum (WLC) fed by the Ti:sapphire laser. A band-pass filter with the central wavelength at 508.5 nm and a full width at half maximum (FWHM) of 10 nm was placed in the WLC beam. For measurements with a time window above 2 ns, the repetition rate was lowered to 2 MHz by a pulse picker. The excitation beam was focused, by a 7.6-cm focal length lens, into a 1-mm quartz cuvette, containing the studied compounds dissolved in either isopropanol, acetonitrile, acetone or toluene. The apparatus functions of the setup were ~5 ps and ~10 ps (standard deviation of a Gaussian function) for the excitation wavelengths of 400 nm and 510 nm, respectively. The PL signal was collected at a ~90 o angle with respect to the excitation laser beam. The polarization of the excitation and PL beams was set at the magic angle (~54.7 o ). Long-pass filters (Thorlabs) with the cut-off wavelength of 420 nm and 515 nm were used to remove the stray light of the excitation beam for 400 nm and 510 nm excitations, respectively, to the polychromator entrance slit. Finally, the PL intensity of the samples was recorded as a function of the wavelength and delay time, producing a PL map. In all measurements, the room temperature was kept at 20 o C.

Transient absorption spectroscopy
The setup of transient absorption (TA) spectroscopy was based on a pump-probe configuration described previously 4 . In brief, an amplified mode-locked Ti:sapphire laser (Legend Elite Duo, Coherent) centered at 800 nm (1 kHz repetition rate) was used. The laser output was split into pump (~90%) and probe (~10%) beams. To obtain the excitation wavelength of 400 nm, the pump beam was frequency-doubled by using a β-barium borate (BBO) crystal. A mechanical translation stage (LS-180, Physik Instrument) with 508 mm excursion was used to delay the probe pulse with respect to the pump pulse. The probe beam was focused into a 2-mm sapphire crystal to generate a white-light (400-850 nm) continuum (WLC). A short-pass filter with a cut-off wavelength of 750 nm placed after the sapphire crystal was used to remove residual fundamental frequency radiation from the WLC. The polarization of the pump and probe beams was linear and set at the magic angle (54.7 o ). Both the pump and the probe beams were focused and spatially overlapped in a 0.2-mm flow cell (Starna Scientific Ltd.), connected to a peristaltic pump (Masterflex, Cole-Parmer) to refresh the sample in the excitation spot. Then, TA of the probe beam in the flow cell was recorded using two different types of detectors, a 350-700 nm compact spectrometer (CCS100/M, Thorlabs) and a silicon photodiode (DET36A, Thorlabs) amplified by a lock-in amplifier (SR830 DSP, Stanford Research Systems). The spectrometer detected the TA spectra in the range of 460-700 nm; however, it had a lower signal-to-noise ratio as compared to the lock-in referenced photodiode. The photodiode detector detected TA signals at a particular probe wavelength using a band-pass filter (e.g., 510 nm or 550 nm, FWHM = 10 nm for both) placed in front of the photodiode. This arrangement improved the sensitivity down to ≅ 10 −5 . Further details of data acquisition and corrections are described in Reference 4.

Single crystal X-ray diffraction
A single crystal of BODIPY/Motor was grown in diethylether and mounted on a cryoloop and placed in the nitrogen stream (Temperature, ~150 K ) of a Bruker-AXS D8 Venture diffractometer. Data collection and processing was carried out using the Bruker APEX3 software 5 . A multi-scan absorption correction was applied, based on the intensities of symmetry-related reflections measured at different angular settings (SADABS) 6 . The structure was solved using SHELXT 7 and refinement was performed using SHELXL 8 . The hydrogen atoms were generated by geometrical considerations, constrained by idealized geometries and allowed to ride on their carrier atoms with an isotropic displacement parameter related to the equivalent displacement parameter of their carrier atoms. No A-or B-level alerts were raised by CheckCIF for the fully refined structure. For bare BODIPY, the shapes of the absorption and PL spectra of bare BODIPY in toluene are quite similar to those in acetone, which exhibit a strong absorption band with a maximum at ~500 nm and a PL band between 500 nm and 600 nm. The spectra reported herein are in line with the previous studies 9 . There is a small blueshift in the absorption and PL spectra, amount to ~7 nm, when changing the solvent from toluene to acetone. This shift is attributed to solvent polarity effects, at which the absorption and PL spectra of BODIPY are blue-shifted with increasing the solvent polarity 9 11 . The PL spectra of BODIPY+Motor in toluene and acetone are similar to those of bare BODIPY, indicating that PL of BODIPY+Motor originates only from BODIPY. curves), indicating that PL originates from the same excited state. Therefore, we conclude that there is no excitation-wavelength dependence on the PL spectral shape of BODIPY/Motor. It is also noticeable that the PL spectra of BODIPY/Motor in acetone show a long tail up to 700 nm. However, time-resolved PL data ( Supplementary Fig. 19) shows that PL in the tail (600-700 nm) decays similarly to that in the strong PL band (500-600 nm) and therefore PL in both regions originates from the same excited state.

PL quantum yields of BODIPY/Motor
Steady-state PL spectra of BODIPY/Motor were recorded in toluene, THF and acetone ( Supplementary Fig. 9). The absorbance of all samples did not exceed 0.1 so that PL reabsorption effect can be minimized. After keeping the samples in the dark for at least 30 min to ensure all metastable isomers proceeded to stable isomers, the PL spectra for BODIPY/Motor in different solvents was measured (Supplementary Fig. 9; blue lines). Then, the samples were irradiated with a 395 nm LED for ~1 min. After the PSS was reached, the samples were transferred to the PL spectrometer within 10 secs and the PL spectra were recorded (Supplementary Fig. 9; orange lines). The PL spectra of BODIPY/Motor after reaching the PSS show the lower PL intensity (at least two folds) than those before irradiation.
Relative photoluminescence quantum yields (PLQYs) were determined following the procedure described in the previous literature 12 . More than five samples with different absorbance were prepared and their PL spectra were obtained before and 10 second right after irradiation with LED light in each solvent. The peak area for each spectrum was calculated and plotted against absorbance. The slope of the linear fit of the plots was compared with that of a standard solution (fluorescein, 0.1 M in NaOH, a PLQY of ~92%) 13 . The relative PLQYs of a sample was calculated by using the following equation: where the subscripts x and st represent sample and standard. , m, and n denote relative PLQY, slope from the plot, and refractive index of the solvent, respectively. Supplementary Table 4 summarized PLQYs of BODIPY/Motor in toluene, THF and acetone before and 10 s right after irradiation.
Supplementary Fig. 9. Steady-state PL spectra of BODIPY/Motor in toluene (a), THF (b) and acetone (c) before and 10 s right after irradiation with a 395 nm LED at 5 °C for 1 min. The excitation wavelengths of all samples were 488 nm. Table 4. Summary of photoluminescence quantum yields (PLQYs; using Equation (1)) of BODIPY/Motor in toluene, THF and acetone before and 10 s right after 395 nm irradiation at 5 °C. All samples were excited at a 488 nm wavelength. Fluorescein (PLQY ~ 92%) was used for a reference sample.

UV/Vis absorption studies on photoisomerization and thermal recovery
The motor rotation of BODIPY/Motor under irradiation condition was studied using UV/Vis absorption spectroscopy and LED light sources. Solution samples were prepared by dissolving The thermal isomerization process was studied at different temperatures (from 5 °C to 30 °C) in the dark after the irradiation. The excitation wavelength was selected at 395 nm, which is close to the largest UV/Vis spectral change ( Supplementary Fig. 10a, 11a and 12a). The rate constant, k, was determined by fitting a 1 st order rate law and the ln(k/T) values were plotted against 1/T ( Supplementary Fig. 13). Thermodynamic parameters for the metastable isomer formation were calculated using the Eyring equation and summarized in Supplementary Table   5.

Photoisomerization quantum yields of BODIPY/Motor
For a typical experiment, a stirred solution (2 mL) of a compound was irradiated from the side to lead to the formation of the respective metastable isomers (M). The spectra were collected following the evolution of the absorption at the wavelength of irradiation, after which the data subsequently fitted using COPASI 4.29 18 following the same approach developed by Stranius & Börjesson 15 Eq. 14 in the original article as follows: Equation (2)  dihedral angles in the gas phase (a) and toluene (b). The ground state (S0), first (S1) and second (S2) excited states are depicted in dark blue, red and green, respectively. The energies relative to BODIPY/Motor stable ground state energy. The purple cones represent the CIs.
Supplementary Fig. 15. Relaxed S1-PES scan of the BODIPY/Motor stable isomer along ϕ1 dihedral angle in acetone and toluene. Supplementary Table 9. Optimized geometry parameters of the S0, S1min, S2min, and CI'S1/S0 and CIS2/S1. The energies are relative to S0min energy of BODIPY/Motor stable isomer (gas phase/acetone/toluene). Note that the CI'S1/S0 is located at the BODIPY folding reaction coordinate starting from stable structure, which is not the case for the metastable. All attempts to optimize CIS2/S1 of metastable isomer crossing in toluene did not converge using the penalty function method. We found a CIS1/S0 structure which shows close similarities with the corresponding geometry of the metastable isomer in acetone.      Fig. 16) in the spectral region between 490 nm and 600 nm (between 520 nm and 600 nm for data of PL maps under 510 nm excitation). The grey solid lines show the best fits to exponential functions convoluted to the Gaussian apparatus function of the respective experimental data. The fitting parameters are listed in Supplementary Table 11. The black arrows depict the PL quenching upon attaching the motor core to BODIPY. For the sake of clarity, the transients of BODIPY+Motor in toluene and acetone are multiplied by 2.

Supplementary
When the motor core is chemically attached to BODIPY (i.e., BODIPY/Motor), PL is shortened to 2.4 − 2.6 ns with toluene used as the solvent (Supplementary Fig. 18a; green dots).
Remarkably, when acetone is used as the solvent, the PL of BODIPY/Motor is shortened even more and exhibits distinct bi-exponentiality with the decaying components having the lifetimes   Fig. 19 shows PL transients in short-wavelength (490-550 nm) and longwavelength (580-700 nm) regions of BODIPY/Motor in acetone under 400 nm excitation. The short-wavelength PL shows a strong signal in the first ~40 ps followed by a long decay. The long-wavelength PL matches well with the short-wavelength PL at the time above 40 ps. These results are in line with the previously discussed results on the mean PL energy ( Supplementary   Fig. 17b), which is redshifted in the early time (up to 40 ps) and remains constant for the longer time. The redshift in mean PL energy is assigned to the excited state relaxation, and therefore PL in the long-wavelength (low-energy) region of 580-700 nm is assigned to the excited state manifolds relaxing after excitation.  Taken from Reference 19 for toluene and from Reference 20 for isopropanol, acetone and acetonitrile.

Transient absorption (TA)
Next, femtosecond transient absorption (TA) spectroscopy was used 4,22,23 Table 6) and PLQY of 2.6% (Supplementary Table 4 Fig. 21e-f) and therefore the timescales of the GSA/SE signals of these compounds can be obtained.
The 510-nm TA trace of bare Motor shows a strong, positive ESA signal decaying to a long-lived offset (share of ~1%), which is attributed to the ground state absorption of the bare Motor in the metastable state ( Supplementary Fig. 21a,d). The early time signal is fitted to a single exponential function with a lifetime ~1.4 ps. This lifetime matches well with that from the previous studies 22,23 and was attributed to the timescale of S1 excited state relaxation while moving the molecular system from the Frank-Condon region towards a CI with the S0 state PES.   Supplementary Table 14. The black arrow depicts the excited state quenching of BODIPY with the attachment of the motor core. The lower amplitude in the TA trace of BODIPY as compared to that of BODIPY/Motor is due to a blueshift in the GSB/SE peak ( Supplementary Fig. 21e-f).  difference is explained by existence of an energy barrier on the S1 state along the motor-rotation coordinate toward the CI point between the S1 and S0 state PESs (CIS1/S0). The height of the energy barrier is lower for more polar solvents (Supplementary Fig. 15). Therefore, the 1~1 0 ps time observed at both 510 nm and 550 nm probe wavelengths is assigned to the S1 excited state relaxation while moving the system to the CIS1/S0 point. In bare Motor, the barrier does not exist which results in much faster relaxation time. al. 25 , the underlying mechanism of this motor functionality was attributed to intermolecular energy transfer from PtTPBP (after excitation and S1-T1 intersystem crossing) to the low-lying triplet excited state of BODIPY/Motor to drive motor rotation 25 . Therefore, we conclude that once the triplet excited state of BODIPY/Motor is populated, motor rotation might occur via accessing the triplet pathway ( Supplementary Fig. 26b; lower graph). Furthermore, the molecular diffusion length of PtTPBP in the triplet excited state is estimated as ~250 nm (Supplementary Section 11.2.3), which is much longer than the average distance between BODIPY/Motor and PtTPBP molecules (~50 nm, using Equation (6)). This result indicates that the intermolecular energy transfer from PtTPBP to BODIPY/Motor is likely mediated by a molecular-diffusion process and further supports above conclusion.   The conclusion from these experiments is the following. The motor rotation can be activated via its triplet state; however, the triplet state does not appear to be involved when the functionalized motor is excited at 505 nm. Therefore, the scenario that the functionality of BODIPY/Motor is activated via the pathway of the triplet excited state, can be reasonably ruled out.

Calculation of molecular diffusion lengths in solution
To examine if energy transfer from a sensitizer (e.g. PtTPBP, PdPc(OBu)8 or BODIPY) to bare Motor in a mixture solution could be mediated by molecular diffusion, we calculated a diffusion length of the relevant molecules in their excited states. If this diffusion length is larger than an average distance between the sensitizer and Motor molecules (which could be estimated from the molar concentrations of constituents in solution, see below), moleculardiffusion mediated energy transfer would likely occur.
The diffusion length of a molecule in the excited state in solution can be calculated using the following equation 28 : where  [a] Estimated from a half of the largest size of the molecular structure [b] Lifetime of the triplet excited state of PtTPBP in THF reported previously in Reference 26.
[c] Lifetime of the triplet excited state of PdPc(OBu)8 in benzene reported previously in Reference 27.
[d] Lifetime of the singlet excited state of BODIPY in toluene obtained from the time-resolved PL measurements (Supplementary Table 11  A plausible assumption is that the excitation energy is dissipated partly into thermal energy which raises the local temperature of the entire sample volume. This increase in temperature after irradiation could cause a change in the ΔOD spectrum of BODIPY+Motor with a spectral shape similar to that resulted by photoisomerization. The temperature change dT of a solution reservoir with the thermal energy added into the reservoir can be derived from the following equation 31 : where = 1.72 g and = 1.7 J • g −1 • K −1 are the mass and specific heat capacity of the solvent (toluene), respectively.
where = 10 mW and = 5 min are the irradiation power and time of the light source, respectively, = 0.87 is the optical density at the irradiation wavelength of the BODIPY+Motor sample, ~95% is the PL quantum yield of bare BODIPY in toluene 9 . By substituting all numerical values into Equations (7) and (8), we obtained ~0.05 K (or 0.05 ℃).
Then we performed ΔOD spectroscopy measurements at different temperatures of the BODIPY+Motor sample, but no irradiation was used. The temperature of the sample holder was varied from 20 o C to 40 o C. ΔOD spectra were obtained by subtracting the initial absorption spectrum at 20 o C from the spectra measured at higher temperatures of the sample.
Supplementary Fig. 28 shows ΔOD spectra of BODIPY+Motor in toluene with the increase in temperature (ΔT) of 1 o C. There is a noticeable change in ΔOD at ~500 nm, which is linearly dependent on ΔT (inset of Supplementary Fig. 28). However, the amplitude of the signal with temperature change is a factor of 20 lower than the change in the absorption spectrum after irradiation with green light (violet curve). Besides, the spectral changes are confined in the spectral region of BODIPY in the BODIPY+Motor mixture; no changes in the region of "motor rotation" (350-450 nm) were detected. This clearly demonstrates that changing the sample temperature (by excitation energy dissipated to thermal energy and/or fluctuation of the sample temperature) does not lead to isomerization of the motor. = 1 o C was obtained from the experimental ΔOD spectrum with = 5 o C and divided by 5. Note that the estimated temperature rise in the focal volume was ~0.05 o C, i.e., a factor of 20 lower than for the red spectrum. The molar concentrations of the motor and BODIPY in BODIPY+Motor were set to be identical at ~1 × 10 −5 M. The inset shows the dependence of ΔOD at 500 nm on ΔT and the black line shows the fit to a linear function. The error bars refer to standard deviation.
Having established that the ΔOD change of the BODIPY+Motor spectrum is not due to the temperature change after irradiation, we move to the next possible scenario, namely diffusion-mediated energy transfer. The molecular diffusion length of BODIPY in the singlet excited state is estimated as ~5 nm (Supplementary Section 11.2.3), which is approximately one order of magnitude shorter than the average distance between BODIPY and motor molecules (~44 nm, using Equation (6)). Therefore, BODIPY in the singlet excited state likely decays to its ground state before approaching the motor molecules. However, if BODIPY is populated to its triplet excited state (a maximum yield of 5% as the fluorescence QY is 95% 9 ), a longer lifetime and thus a longer diffusion length of BODIPY in the excited state can be obtained. If this is the case, motor rotation in the BODIPY+Motor mixture occurs via the triplet pathway after TTET from BODIPY. Therefore, even though we could speculate that the motor in the BODIPY+Motor mixture functions under irradiation with 505-nm green light via accessing the triplet pathway, this channel is much less efficient than the one arising from the synergetic effects, i.e., direct excitation of the BODIPY/Motor system.

Circular dichroism spectroscopy
Circular dichroism (CD) spectroscopy measurements of BODIPY/Motor were performed after the enantiomers (R and S) were successfully separated by supercritical fluid chromatography (SFC). Without SFC of the motor, the change in the CD spectra cannot be observed because of the coexistence of both R and S enantiomers in solution, e.g. bare motor core ( Supplementary   Fig. 29e).
The samples of BODIPY/Motor with the R and S enantiomers (hereafter, denoted as Rand S-BODIPY/Motors, respectively) were prepared by dissolving them in acetone (0.9 × 10 -5 M). The CD spectra of R-and S-BODIPY/Motors were measured in three different states: before irradiation, 10 secs and 10 min after reaching the PSS by irradiation with 395-nm light ( Supplementary Fig. 29a-c). In the latest state, the sample was kept at 30 o C in dark to ensure all metastable isomers progressing to their second, identical stable isomers via THI. The CD spectra of R-and S-BODIPY/Motors before irradiation ( Supplementary Fig. 29a) shows a mirror image in circular dichroism, indicating the enantiomeric couple of BODIPY/Motor at the stable state. Interestingly, the spectra exhibit two clear CD peaks at ~390 nm and ~500 nm, which lies on the absorption region of the motor core at the stable state and BODIPY moieties, respectively. This result indicates that chirality from the chiral motor core is transferred to the achiral BODIPY moiety. Performing similar experiments on bare BODIPY, we observe no change in the CD spectra ( Figure S12d), indicating that BODIPY is an achiral chromophore, thus supporting the above conclusion.
After irradiation to reach the PSS, the CD spectra of R-and S-BODIPY/Motors ( Supplementary Fig. 29b) shows the opposite CD signs as compared to those of the spectra before irradiation. This inversion of the CD sign is attributed to the inversion of the helical chirality at the motor core, which has been observed previously 14 . The consequence of the helical chirality inversion is observed, which the CD peak associated to the absorption of the motor core is redshifted to, ca., 430 nm. In contrast, the CD absorption maximum associated to the absorption of BODIPY remains at ~500 nm but opposite in sign, indicating that this CD peak can be exploited as a visible-wavelength reporter of the chirality inversion in BODIPY/Motor. and NMR (Supplementary Section 4). Supplementary Fig. 29. (a-b) Circular dichroism (CD) spectra of R-and S-BODIPY/Motors before irradiation (a), 10 secs (b) and 10 min (c) after reaching the PSS by irradiation with a 395-nm LED in acetone. The samples before irradiation and 10 sec after reaching the PSS was kept at 5°C and afterward kept to 30°C for 10 min. (c-d) CD spectra of BODIPY (d) and racemic rac-Motor (e) in acetone before and after irradiation with a 395-nm LED.